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Book Review - At the Water's Edge

The full title of this book is At the Water's Edge : Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea. It was written by Carl Zimmer, and as the long title suggests, is all about those two dramatic transitions of life evolving into such distinct environments. This book was great - one of the best non-fiction books I've read in a while. It was just the right blend of story telling, concepts, and evidence, and made for a very compelling read. In fact, I think I finished it in less than a week.

When I reviewed another book by Zimmer, the Smithsonian Intimate Guide to Human Origins, I commented that it wasn't very in depth. At only 176 pages, much of them filled with photos and illustrations, it was a little light on commentary. At the Water's Edge is very different in this regard. It's 304 pages, filled with small print, with only enough diagrams as are needed to illustrate a few key points. It's not a tome, by any means, but it certainly provides Zimmer with enough space to do this subject justice.

The book is divided into basically two halves - the first dealing with the transition from lobe finned fish to early tetrapods, and the second half dealing with the transition from mesonychids to dolphins and whales. As would be expected, both halves deal with the specifics of each of those cases - transitional forms that have been discovered, environmental pressures that would drive the transition, etc. However, mixed throughout the entire book are also sections on general theory. There's a nice section on development in the beginning, covering such topics as Hox genes and non-genetic factors; he describes exaptation; there's another section on cladistics; as well as sections on many other concepts related to evolution.

I learned quite a bit by reading this book. Even though I was already familiar with much of the general theory, Zimmer presented it in ways that made me think of things differently. He also introduced a few concepts, such as the evolutionary "quit point," that I hadn't thought of much before. Still, where I learned the most was in those specifics of the transitional forms between fish and tetrapods, and land mammals and whales.

I'll give one example of something very interesting I learned from this book. (In fact, this was the very first passage of the book that I read, when I first got it and was just thumbing through to see what it was like.) At some point, our ancestors must have developed lungs to breathe air, obviously. When we look at the world around us, most fish today cannot breathe atmospheric air - they rely on their gills to get oxygen from water, but also have organs similar to lungs called swim bladders, which they use to regulate their buoyancy. From that observation, you may be tempted to think that lungs are a modified swim bladder, which perhaps evolved to allow fish to survive in swamps or other oxygen poor environments. After all, what need would an ocean going fish have of lungs? I know that's what I had thought, but as it turns out, it's almost certainly wrong.

When you look at a phylogenetic tree of fish & tetrapods (such as the one on this page, which also has an excellent discussion on this topic), it becomes apparent that it's only one group of fish that has true swim bladders, while a few closely related groups have bladders that are open to air exchange, and most groups have lungs. It seems highly unlikely that swim bladders would have independently evolved into lungs in so many lineages, so the simplest explanation is that lungs came first, and later evolved into swim bladders in that one group. The thing is, that one group, the teleosts, or ray-finned fish, has become the most successful group of vertebrates on the planet, with over 20,000 extant species. Their success has biased our perception of what is normal concerning swim bladders vs. lungs.

After studying the phylogenetic tree, a person may still be left wondering what use lungs would be to fish. As it turns out, they can be quite useful. Obviously, there are all the living fish that have lungs (such as, as its name implies, the lungfish), or ray finned fish that have independently evolved means of breathing atmospheric oxygen (such as those staples of pet stores, bettas). Oxygen supplies are not as consistent in water as in the air, so it could be very advantageous for a fish to be able to get oxygen that way. Even in the ocean, oxygen levels can vary. There's another reason that I'd never even considered, which has to do with the circulatory system of a fish.

A fish's blood is pumped from the heart, to the gills where it picks up oxygen, through the body, and then back to the heart again. By the time the blood gets back to the heart, the other tissues have already absorbed most of the oxygen and there's not much left for the heart. So perhaps, lungs were a way for fish to supplement their oxygen intake, and be sure that the heart didn't become oxygen starved (more info can be found on this in an article by Zimmer in Natural History magazine, available online).

Finally, one may ask, if lungs were so useful to early fish, why would the teleosts have lost them? One explanation Zimmer offered, is that perhaps once airborn predators, like pteranodons, evolved, coming to the surface to grab a gulp of air became riskier behavior, and was no longer worth it. Perhaps it was because buoyancy control was that useful. There's also the fact that some teleosts have evolved a new way to keep their heart oxygenated - with coronary arteries that run from the gills straight to the heart. Maybe it was all these factors combined that caused teleosts to lose the ability to breath with their lungs/swim bladders, or perhaps some other reason we haven't thought of, yet. (Just as a note, sharks and rays diverged from bony fish around 420 million years ago. Sharks and rays have neither bladders nor lungs, indicating that the organs developed after that split. You'll also notice that means a gold fish is more related to us mammals than to sharks.)

In that introductory paragraph, I mentioned story telling. Zimmer does not simply present the theories and evidence for these transitions. He talks about the people involved in the discoveries, going all the way back to Sir Richard Owen (the man who coined the term dinosaur) all the way up to meeting Ted Daeschler on a rainy afternoon in Pennsylvania. He describes the personalities involved, and their contributions to our current understanding. You can see how these theories have been refined over time.

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